Fluorinated polymers containing sulfonyl fluoride functional groups are known in the prior art as precursors for a class of ion exchange fluorinated polymers generally referred to as “ionomers”.
Due to their ionic properties, fluorinated ionomers are suitable in the manufacture of electrolyte membranes for electrochemical devices such as fuel cells, electrolysis cells, lithium batteries.
Fuel cells are electrochemical devices that produce electricity by catalytically oxidizing a fuel, such as hydrogen or methanol. Among known fuel cells of particular interest are proton exchange membrane (PEM) fuel cells which employ hydrogen as the fuel and oxygen or air as the oxidant. In a typical PEM fuel cell, hydrogen is introduced into the anode portion, where hydrogen reacts and separates into protons and electrons. The membrane transports the protons to the cathode portion, while allowing a current of electrons to flow through an external circuit to the cathode portion to provide power. Oxygen is introduced into the cathode portion and reacts with the protons and electrons to form water and heat.
An important requirement for the long-term functioning of a PEM fuel cell is the ability of the membrane to maintain suitable water content in the membrane itself to ensure the required level of ion conductivity. It is important that, while in operation, the membrane maintains a high proton transport capability and efficiently transfers water generated during the cell operation from one side of the membrane to the other.
Ionomers deriving from fluorinated polymers containing sulfonyl fluoride functional groups having a heat of fusion not exceeding 4 J/g have shown to possess advantageous properties in terms of both proton and water transport capability. In general, the higher the amount of monomeric units comprising sulfonyl fluoride functional groups contained in the polymer, the lower the heat of fusion (and consequently the crystallinity of the polymer) and the higher the ionic conductivity of the membranes obtained therefrom.
Said polymers however have the drawback that when transformed into ionomeric membranes and used in e.g. a fuel cell they tend to progressively lose weight during the lifetime of the cell.
It would therefore be desirable to have sulfonyl fluoride polymers having a heat of fusion not exceeding 4 J/g and having an increased durability when transformed into a membrane for a fuel cell application.